Such a vehicle typically has a front-end engine compartment containing a powerplant, such as an internal combustion or hybrid-electric engine, that powers the vehicle. The engine is part of the vehicle's powertrain which in a rear wheel drive vehicle comprises a transmission coupled to the engine, a driveshaft coupling the transmission to a differential of a rear axle, and driven road wheels at the ends of the axle.
The power take-off unit has an input that is coupled either to the engine or to the transmission. While there are various types of power take-off units, they share a common characteristic of having a mechanism that is controlled in some manner for selectively coupling and uncoupling the unit's output to and from the unit's input.
A typical front-end engine compartment is essentially closed along its sides and top, but is partially open at the front to allow air to pass through a radiator that is disposed between the frontal opening and the engine to allow air to pass through a radiator in the engine cooling system and the engine compartment and eventually to exit by passing downward and under the vehicle body. Access to the engine compartment interior is available by opening a hood of the vehicle.
Although access to the engine compartment and its contents is restricted when the hood is closed, it may be possible for the engine to be cranked and started by operating the usual ignition switch in the occupant compartment of the vehicle while the hood is open. It may also be possible for the hood to be opened while the engine is running. Opening the hood to access the engine, either when running or when being cranked and started by the ignition switch, may be important for diagnostic or other purposes.